Method for adapting a filter medium to predefinable parameters and preferably a filter medium produced according to said method

ABSTRACT

The invention relates to a method for adapting a filter medium to predefinable parameters, such as electrostatic charge, flow-rate behaviour, dynamic pressure behaviour or differential pressure behaviour, fraction filtration efficiency and preservation of a constant quantity and quality of fluid additives. Said method is characterised in that at least one of the aforementioned parameters is adapted by the targeted influencing of the surface energy of the filter medium ( 18 ).

The invention relates to a method for adapting a filter medium to predefinable parameters, such as

-   -   electrostatic charge,     -   flow-rate behavior, dynamic pressure behavior, or differential         pressure behavior,     -   fraction filtration efficiency, and     -   preservation of a constant quantity and quality of fluid         additives.         The invention also relates to a filter medium produced         preferably according to this method.

A method and means for rendering germproof air filter materials hydrophobic, which are preferably used in the areas of anesthesia and surgery, is known from DE 38 19 583 A1. The hydrophobic means in question, with which the surface of the aforementioned filter materials is treated, includes silicones and silicone compounds. These are brought into contact with the filter material by spraying or dipping processes or vapor deposition. The hydrophobic means in the form of a solution or emulsion or in pure form is said to improve the air flow of the air filter material, to be physiologically harmless and to meet medical-hygienic requirements.

A fuel filter or tank filler neck insert is known from DE 199 32 383 A1 as a molded part having a body made of an electrically non-conductive plastic, the surface of which is coated entirely or partially with a striped, linear, lattice or net-like structure made of an electrically conductive material, preferably in the form of an electrically conductive polyacetate composition, wherein different areas of this structure are selectively electrically conductively connected to one another. This molded part is advantageously produced according to the monosandwich method or the sequential injection molding method.

A filter element for filtering liquids, in particular, in the form of an oil filter, is known from DE 197 35 993 A1, having a zigzag-like foldable filter strip made of a non-woven filter material. The non-woven filter material may contain metal fibers, glass fibers and/or polyester fibers, wherein the zigzag-like foldable filter strip is produced from pre-stamped panels of non-woven filter material or from a rolled strip of non-woven filter material. The filter element, if it is constructed of glass fiber non-woven materials, may be advantageously laminated or coated on one or both sides with polyester fibers. In this way, a type of grooving or other imprinting may be easily carried out and the sensitive glass fiber non-woven material may be processed to form a star-like pleated filter element without additional protective non-woven material on the inflow side or the outflow side and, if applicable, also with no backing.

Thus, it is state of the art to modify, in particular, to finish known filter media for air, fuel and oil filtration using corresponding coating methods, to thereby create improved characteristics for the respective filter medium. However, this does not include a targeted adaptation to pre-definable desired parameters; instead, depending on the parameter or requirement profile, the respective filter medium is coated more or less randomly and in a trial and error fashion with materials from which there is hope of corresponding improvements in the operation of the filter medium.

Based on this prior art, the object underlying the invention is the targeted adaptation of the filter medium to the exemplary parameter values described above. Such an object is achieved by a method having the features of Claim 1 and a filter medium according to the feature configuration of Claim 9, each in its entirety.

The adaptation, because it is made according to the characterizing portion of Claim 1 to at least one of the aforementioned parameters by a targeted influencing of the surface energy of the filter medium, no longer takes place in a random and experimental fashion, but rather targeted, by considering the respective surface tension of fluids, such as hydraulic oil, fuel or air, and/or of solid bodies, such as filter element materials (filter medium). This consideration of the surface energy is based on the finding that the surface energy of the respective filter medium can be divided into a disperse fraction and a polar fraction, and that by appropriate selection of coating methods and of coating and infusion media, the respective proportion of the disperse and polar fractions are adjustable relative to one another in such a way that the filter medium treated in this manner is adapted to the predefinable parameters optimal for its operation.

To determine the respectively prevailing surface energy, in addition to its disperse or polar fractions, a contact angle measurement is preferably used, specifically, on the filter medium itself and/or on the fluid flowing through the filter medium. If, for example, the determining parameter for the filter medium is the electrostatics, in the sense that an electrostatic charge, which may damage the filter medium material, is avoided, the magnitude of the difference between the surface tension of the hydraulic oil to be filtered and the surface energy of the filter material or filter medium is the crucial factor in the charging of the oil. It has been shown in such case that the more similar the materials are in terms of their surface energy, the less oil charging is expected during operation. Thus, based on the tribo-electrical series known for materials, the material of the filter medium, in terms of the surface energy to be determined, must be designed in a way similar to the hydraulic oil to be filtered and the coating material used.

The flow rate behavior, which is reflected in the differential pressure, may likewise be decisively influenced by adjusting the surface energy of the filter medium. As the hydrophobicity of the material increases, so too does the so-called bubble point, i.e., the pressure that is required to push the fluid for the first time into the pores of the filter medium, as based on the so-called Washburn equation. If the material is wetted, the flow rate resistance decreases as a result of the “repulsion of the liquid” by the hydrophobic coating at the boundary surface between the filter medium and the liquid. Areas of application include primarily air filters, in addition to hydraulic filters. Comparable considerations also apply if the intention is to decisively influence the so-called water entry pressure (dynamic pressure) by adjusting the surface energy of the filter medium.

The fraction filtration efficiency or the coalescence factor as an additional parameter may also be influenced by adjusting the surface energy of the filter medium. As the hydrophobicity of the filter medium (filter material) rises, the fraction filtration efficiency is able to increase significantly. The filter medium is then significantly more resistant to penetration of liquid droplets from the air, so that such droplets can be more easily separated from an air flow. This effect may be employed as a barrier, for example, for the separation of oil mist, in the case of air filters, ventilation filters or so-called coalescence filters.

In preserving a constant quantity and quality of fluid additives, the undesirable separation of oil additives from the fluid may be avoided, which counteract oil ageing, for example, specifically, again by manifest adjustment of the surface energy. Additives dispersed in oil present as non-dissolved substances, for example, in the form of silicon droplets as anti-foam additives for the oil, are separated on the surface of the filter medium as a result of boundary surface effects, such as Van-der-Waals forces and the like. These effects are reduced, if not outright prevented, by a targeted change of the surface energy. Accordingly, surface modified filter media may be “smoother” during filtration and help to avoid intrusions of oil in the additive packet.

Based on the targeted split of the surface energy into polar and disperse fractions, theoretical combinations for complete wetting may be represented by predefinable reference curves (wetting envelope). Based on these, it is possible to reliably estimate the wetting behavior or other effects of the filter medium in relation to the liquid to be wetted.

Additional advantageous embodiments of the method according to the invention are the subject matter of the dependent claims.

The subject matter of the solution according to the invention is also a filter medium, wherein the filter medium is produced at least in part from glass fibers, which are encased in a non-bonding manner at least partially by polymerized silicon dioxide molecules, or the glass fibers form a covalent bond with fluoropolymers, polyethylene or polypropylene. In addition, the following coatings have proven advantageous for modifying the surface energy of the filter medium, for example, in the form of polydimethylsiloxanes (PDMS) or in the form of poly-3,4-ethylenedioxythiophene (PEDOT).

The solution according to the invention is explained in greater detail below with reference to various embodiments. In the drawings, depicted schematically and not to scale,

FIG. 1 shows the structure of the device for finishing the filter material or filter medium by means of a sol-gel spray method;

FIG. 2 shows the structural formula for a molecule of the silicon dioxide used for coating;

FIG. 3 shows a detail of the silicon dioxide molecules cross-linked with one another;

FIGS. 4a, b show the respective contact angle relative to water and oil by changing the surface energy of the filter medium by means of the respective coating method according to the invention;

FIG. 5 shows the structure of the device for finishing the filter medium by separating monomers from the gas phase; and

FIG. 6 shows a graphic depiction of a wetting envelope scheme for filter materials.

Filter media, in particular, in pleated form, and filter elements constructed therefrom, in particular for the filtration of fluids, such as hydraulic oil, are sufficiently known and described in detail, for example, in the applications for patent rights DE 10 2013 000 932, DE 10 2013 000 933, DE 10 2013 000 934, DE 10 2013 000 939, DE 10 2013 020 752, etc. of the property rights owner, so that these are not discussed in further detail at this juncture and the filter medium according to the invention, along with its respective method of production, will be described below only insofar as it concerns the finishing, in particular, by the targeted influencing of the surface energy of the filter medium.

Filter media, even when they are constructed in multiple layers and include protective non-wovens and support screens, regularly exhibit a high percentage of glass fibers for the filtration of fluids, such as air, gases, process water, fuel and lubricants as well as hydraulic oil, etc. The aforementioned glass fibers for filtration, in addition to silicon dioxide, also consist of other oxides, such as aluminum (Al), barium (Ba), boron (B), calcium (Ca), potassium (P), magnesium (Mg), manganese (Mn) and zinc (Zn), in various compositions. In addition to the commonly known borosilicate glass, special glass compositions are also used, such as low alkali, ferrous glass (E-glass); acid-resistant high carbon glass (C-glass); low heavy metal glass; low boron glass or quartz glass. The components are mixed and melted together and cooled to form glass beads or pellets. By re-melting and pressing the liquid glass through nozzles, the glass is stretched and deposited on a metal screen. As a further type of production, glass rods are heated and stretched by means of a glass flame and subsequently deposited on a screen.

The resultant glass fibers are further processed in a paper machine to form filter material. In the process, the glass fibers are dispersed in water in a pulper. To lend strength to the glass fibers, the fibers are bonded using an acrylate or epoxy resin. Before finally being deposited, such a binder is placed on the wet paper web under a vacuum on a screen or at the end of the screen. The glass fiber medium is subsequently dried in a drying stage and is suitable for use as a filter material.

Another filter material used very frequently for a filter medium consists of so-called melt-blown fibers. Melt-blown fibers are regularly produced in a process, in which non-woven materials are produced directly from granulate. A specialized weaving method is used in combination with high-velocity hot air in order to produce fine fibered materials having different structures. Granulate melted in an extruder is fed directly into a nozzle block via a spinning pump. The polymer, once it has exited through the nozzle tips, is drawn by means of compressed process hot air. The resultant microfiber non-woven is deposited on an air-permeable sieve belt. To produce the laminates, the melt-blown unit is extended by an uncoiler in front of and behind the nozzle. A calander bonds the fed materials to one another. Potential feed materials include polypropylene (PP), polybutylene terephthalate (PBT), polyethylene (PE), polyamide (PA) and other comparable plastics. Each filter material produced in this way is then pleated with additional filter layers (wire mesh, non-wovens, nets) in knife pleating machines or rotary pleating machines and further processed to form a complete filter element. The production methods described above likewise form part of the prior art.

In the following, it will now be described how such known fiber material of the filter media is correspondingly finished. As a departure from the production solution for a filter medium described above, at least one finishing step for the filter material or filter medium is carried out before the medium is pleated. Thus, the completely fabricated glass fiber filter material is finished prior to pleating using a spray technique. For this purpose, a so-called sol-gel spray method is carried out in an appropriate device according to the illustration in FIG. 1. For this purpose, the glass fiber medium present on a roll is wound up and down between two roller units 10, 12 in the direction indicated by the arrow. One or multiple spray bars 16 are located in the transport path 14 between the two roller units 10, 12, by means of which the filter material 18 is sprayed with a coating material according to the vertical arrows depicted. The wet chemical finishing applied to the glass fiber paper in this manner is dried in a connecting drying path 20 and subsequently rolled up again via the downstream roller unit 12. The filter medium rolled up in this manner may then be fed directly to the previously mentioned pleating process for producing the complete element. Instead of the aforementioned spraying process, the coating material could also be applied by means of an immersion bath (not depicted).

Preferably polymerized silicon dioxide molecules are used as a coating material for this wet chemical sol-gel method of application. The molecules may be used in combination with other chemical substances, for example, iron, magnesium, calcium, fluorine, etc. and, in particular, as further modifications of the oxides of silicon. The thickness of the coating may be individually adapted to the filtration task from several nanometers up to a micrometer by the adaptation of the parameters during the application process.

Whereas the individual silicon molecule is reproduced in the manner of a structural formula in FIG. 2, FIG. 3 shows the cross-linked arrangement of this type of silicon dioxide (SiO₂) molecules. Due to the electro-negative oxygen groups depicted in FIG. 2, the aforementioned cross-linking occurs as a result of Van der Waals forces between the individual molecular groups. For such a bonding, the molecules were initially dispersed in a liquid carrier fluid, for example, in the form of water. The carrier fluid is removed in the drying path 20. The molecules remaining as the coating are then, as previously explained, self-cross-linking, which means that they become geometrically aligned and form a coherent structure, as a result of which the glass fibers or melt-blown fibers are virtually completely encased and therefore coated, without a chemical bond forming between the fiber furnish material and the coating material.

The main focus of attention of the present coating is in the increase in hydrophobicity (oleophobicity). The surface energy may be determined by measuring the contact angle CA (sessile drop) (for example, using the Owens, Wendt Rable, Kälbe, Wu methods, etc.). Additional test methods involving the determination of the surface energy are the Du Pont test methods or the 3M water repellency test.

When measuring the contact angle, it is possible to determine indirectly the surface energy or surface tension of the filter material. Untreated filter materials made of glass fiber plastics generally have a contact angle CA relative to water (H₂O) and/or oil of <90°, which is shown in FIG. 4a for a corresponding drop 22, which wets a horizontal surface 24. With the coating according to the invention described above, however, the contact angle CA relative to water and oil can be increased significantly higher than 90°, as per the illustration according to FIG. 4 b.

FIG. 5 shows another finishing process for the aforementioned filter material, this time by separating fluoropolymers from the glass phase. The completely fabricated glass fiber filter material is finished from the gas phase separation by means of a reaction taking place in a vacuum, again prior to the pleating step. In this process, the glass fiber medium present on a roll is again wound down and up again between the two roller units 10, 12, as indicated by the direction of the arrow. The transport path 14 is located this time in a vacuum chamber 26, in which the filter material 18 is later treated with a coating material.

The entire device system is moved into a low pressure vacuum and an initial plasma activates the surface by forming locations of free radicals on the surface of the substrate in the form of the glass fiber material. A coating material made of monomer gas introduced into the chamber 26 forms covalent bonds on the surface of the substrate or filter material 18 at the locations of the free radicals. Thus, the coating material is anchored on the surface and is extremely resistant. A subsequently produced pulsating plasma 30 activates the applied coating monomer and causes the polymerization of the material at the surface of the filter medium 18. The internal pressure of the chamber is then returned to ambient pressure and the filter material is coated so as to be hydrophobic/oleophobic. The monomer gas, before entering the actual vacuum chamber 26, is stored in an antechamber 32 and the vacuum chamber 26 may be fitted on its output side with a type of exhaust gas post-treatment device 34. Since the plasma separation can release very high amounts of energy, a box-shaped cooling device 36 is attached to the underside of the transport path 14 in question for cooling the filter medium or filter material 18.

Preferably all fluoropolymers are considered as coating material for this method of finishing from the gas phase. However, the following monomers in particular, are suited to be polymerized as coating material in this method:

-   -   Ethylene (E)     -   Propylene (P)     -   Vinylfluoride (VF1)     -   Vinylidene fluoride (VDF or VF2)     -   Tetrafluoroethylene (TFE)     -   Hexafluoropropylene (HFP)     -   Perfluoropropylvinylether (PPVE)     -   Perfluoromethylvinylether (PMVE)     -   Chlorotrifluoroethylene (CTFE)

Final products belonging to the group of fluoropolymers as coating material are, in particular:

-   -   PVF (polyvinyl fluoride)     -   PVDF (polyvinylidene fluoride)     -   PTFE (polytetrafluoroethylene)     -   PCTFE (polychlorotrifluorethylene)     -   PFA (perfluoroalkoxy polymer)     -   FEP (fluorinated ethylene-propylene)     -   ETFE (polyethylene tetrafluoroethylene)     -   ECTFE (polyethylene chlorotrifluoroethylene)     -   FFPM/FFKM (perfluorinated elastomer [perfluoroelastomer])     -   FPM/FKM (fluorocarbon [chlorotrifluoroethylene vinylidene         fluoride])     -   PFPE (perfluoropolyether)     -   PFSA (perfluorosulfonic acid)

The thickness of the coating may again be adapted individually from several nanometers up to a micrometer by the adaptation of the parameters during the application process. Comparable contact angles CA also result, as they are shown in FIGS. 4a and 4b . The last mentioned coating is suitable on the whole in the area of filtration of hydraulic oil and lubricating oil as well as in the area of air filtration and fuel filtration.

In addition to the plasma-enhanced chemical gas phase separation, as introduced above, other chemical gas phase separations may also be used. Physical gas phase separations are equally suitable, in plasma-enhanced form as well. In addition to the coating materials described above, polydimethylsiloxane (PDMS) has also proven very reliable, as well as polythiophene of various types. Fibers made of plastic materials, including cellulose materials, which may also be impregnated with phenolic resin, may also be used in place of the glass fiber materials and the melt-blown fibers.

For an exact determination of the surface tension of liquids and solid bodies, the former is divided into a polar fraction and a disperse fraction. Whereas the polar fraction of the surface tension results from a dipole-dipole interaction and from the hydrogen group bonding or the Lewis-acid-base interaction, the disperse fraction of the surface tension is properly characterized by a Van der Waals interaction.

FIG. 6 shows a so-called wetting envelope for filter materials, divided into a polar fraction of the surface energy in the y-direction and the disperse fraction of the surface energy plotted in the x-direction. Each fraction of the surface energy is determined in accordance with the Kälble method (OWRK), wherein the uppermost delineated curve 38 with its single measured values represented by dots relates to a standard filter material. The underlying curve 40 relates to a coated filter material, wherein polydimethylsiloxane (PDMS) is used as the coating material. The underlying third curve 42 relates to a coated filter material, wherein in this case a gas phase separated fluoropolymer was used. A predefinable modification of polar and disperse fraction of the surface tension is possible, depending on the base material of the filter medium and the respective fluid to be filtered with the filter medium that includes this coating material, which may also be applied as a multilayer composite, and that includes the layer thickness thereof.

By practical trials or by basic theoretical considerations associated with each filtration task along with the desired parameter, for example, it is possible to determine a favorable curve profile of the surface tension, and then, based on these findings obtained, to select the finishing process for any arbitrary filter material or filter medium in such a way that the predeveloped wetting envelope-curve (FIG. 6) results in cooperation with the liquid or fluid to be filtered. This notion of the specific finishing by adjusting the polar and disperse fractions of the surface energy for filter materials and filter media as such has no equivalent in the prior art. 

1. A method for adapting a filter medium to predefinable parameters, such as electrostatic charge, flow-rate behavior, dynamic pressure behavior, or differential pressure behavior, fraction filtration efficiency, and preservation of a constant quantity and quality of fluid additives, characterized in that the adaptation is made to at least one of the aforementioned parameters by a targeted influencing of the surface energy of the filter medium (18).
 2. The method according to claim 1, characterized in that the surface energy of the respective filter medium (18) is subdivided into a disperse and a polar fraction, and that by appropriate selection of coating methods and of coating media and infusion media, the respective proportion of the disperse and polar fractions is adjusted relative to one another in such a way that the filter medium (18) treated in this manner is adapted to the optimal predefinable parameters.
 3. The method according to claim 1, characterized in that the respectively prevailing surface energy, together with its disperse or polar fractions, is determined by means of a contact angle measurement (CA), specifically on the filter medium (18) itself and/or the fluid flowing through the filter medium (18).
 4. The method, preferably carried out according to claim 1, characterized in that a coating material is used as the application medium for the filter medium (18), which encases the fibers of the filter medium to be coated in a self-cross-linking manner and/or forms covalent bonds with free radicals of the fiber material of the filter medium in such a way that the coating material is polymerized on at least portions of the surface of the filter medium (18).
 5. The method according to claim 1, characterized in that it is designed as a wet chemical coating method, in particular, in the manner of a sol-gel spray method, or chemical gas separation method (CVD), or physical gas separation method (PVD).
 6. The method according to claim 1, characterized in that silicon dioxide molecules, in particular, polymerized, are used for the wet chemical coating method.
 7. The method according to claim 1, characterized in that the silicon dioxide molecules in combination with other chemical substances, such as iron, magnesium, calcium or fluorine, are present as further modifications of the oxides of the silicon.
 8. The method according to claim 1, characterized in that monomers are used for the respective gas separation method, which are polymerized to form fluoropolymers, polyethylene or polypropylene.
 9. A filter medium, preferably producible according to one of the preceding methods according to the feature configuration of claim 1, wherein the filter medium (18) is produced at least partly from glass fibers, characterized in that the glass fibers are encased at least partially by polymerized silicon dioxide molecules in a non-bonding manner or that the glass fibers form a covalent bond with fluoropolymers, polyethylene or polypropylene.
 10. The filter medium according to claim 9, characterized in that the selected coating thicknesses at least for the glass fiber material of the filter medium (11) are selected between several nanometers up to 1 micrometer depending on the filtration task. 